Various compounds have been suggested as photoinitiators for photochemically induced cationic polymerizations of such materials as epoxy resins, cyclic ethers, cyclic esters, such as cyclic formals and cyclic acetals, polyvinyl acetals, phenoplasts, aminoplasts, lactones and silicones.
Along these lines, see U.S. Pat. No. 4,161,478 to Crivello, and Watt, et al., "A Novel Photoinitiator of Cationic Polymerization: Preparation and Characterization of Bis[4-(diphenylsulfonio)phenyl]-sulfide-Bis-Hexafluorophosphate", Journal of Polymer Science: Polymer Chemistry Edition, Vol. 22, p. 1789, 1980 John Wiley & Sons, Inc.
Certain sulfonium and iodonium salts have been suggested as the initiators for such cationic polymerizations. Additional discussions concerning these previously suggested sulfonium and iodonium salts can be found, for instance, in Pappas, et al., "Photoinitiation of Cationic Polymerization. III. Photosensitization of Diphenyliodonium and Triphenylsulfonium Salts", Journal of Polymer Science: Polymer Chemistry Edition, Vol. 22, pp. 77-84, 1984 John Wiley & Sons, Inc.; Crivello, et al., "Photoinitiated Cationic Polymerization with Triarylsulfonium Salts", Journal of Polymer Science: Polymer Chemistry Edition. Vol. 17, pp. 977-999, 1979 John Wiley & Sons, Inc.; Crivello, et al., "Complex Triarylsulfonium Salt Photoinitiators. I. The Identification, Characterization, and Syntheses of a New Class of Triarylsulfonium Salt Photoinitiators", Journal of Polymer Science: Polymer Chemistry Edition, Vol. 18, pp. 2677-2695, 1980 John Wiley & Sons, Inc.; and Crivello, "Cationic Polymerization--Iodonium and Sulfonium Salt Photoinitiators", Advances in Polymer Science, Series #62, pp. 1-48.
Various photoinitiators are also discussed in U.S. Pat. Nos. 4,175,972; 4,442,197; 4,139,655; 4,400,541; 4,197,174; 4,173,476; 4,069,055 and 4,299,938 and European patent applications 84/0094914 and 84/0126712.
Although various of the prior art onium salts are very efficient photoinitiators for a wide range of cationic polymerizable materials, such suffer from the deficiency that ultraviolet light absorption in the 300-400 nanometer range is lacking. This range, however, represents an important range given off by mercury lamps. In order to render these onium salts usable in such ranges, it is necessary to add a photosensitizer that absorbs light in the 300-400 nanometer range to the onium salt. Examples of previously suggested photosensitizers are perylene, anthracene and various alkyl derivatives of anthracene.
Although these materials have been effective as photosensitizers, their use has been restricted because such are either unsoluble or exhibit only very limited solubility in common organic solvent. This problem of solubility precludes use of these photosensitizers in thin films (e.g. thickness&lt;10 microns) applications.